Binder system for electrodeposition coating

ABSTRACT

Binder systems suitable for electrodeposition coating and based on basic, thermally crosslinkable epoxy or polyurethane resins as binders (B), containing from 0.001 to 1% by weight of a polymer (I) whose predominant components are ethyl acrylate and/or n-propyl acrylate, based on the overall solids content of the binder system.

DESCRIPTION

The present invention relates to new binder systems which are suitablefor electrodeposition coating and are based on basic, thermallycrosslinkable epoxy or polyurethane resins as binders (B), containingfrom 0.001 to 1% by weight of a polymer (I) whose predominant componentsare ethyl acrylate and/or n-propyl acrylate, based on the overall solidscontent of the binder system.

The invention also relates to new pigment paste formulations forelectrodeposition coating which contain the polymers (I). The inventionrelates, furthermore, to a method of electrodeposition coating using thenew binder systems, to the use of polymers (I) as an additive toelectrodeposition baths for improving the surface quality of thearticles coated by means of electrodeposition, and to the coatedarticles themselves.

It is generally known to coat metallic articles with the aid of cathodicelectrodeposition, a process of which use is made, in particular, in themotor vehicle industry. The electrodeposition coating systems used forthis purpose generally contain, as fundamental components, a binder,also called base resin, a crosslinking component and a pigment paste.

Epoxy and polyurethane resins are known, as binders, in numerousconfigurations. EP-A 261 385 describes binder combinations for cathodicelectrodeposition coating which crosslink by means of external agentsand are based on an epoxy resin, containing hydroxyl and amino groups,and on a poly(meth)acrylate resin containing amino groups. Also knownare coating systems which contain, in addition to the principalcomponents, auxiliaries which reduce the surface tension in the coatingmaterial and thus improve the surface quality ofelectrodeposition-coated articles. The addition of these auxiliaries isnecessary, in particular, if the binder systems used have a tendencytoward problems in coating flow and toward the formation of craters inthe coating film and give rise to adhesion problems in respect of thetopcoats applied subsequently. The binder systems known to date whichcontain silicone oils, polyether-urethanes (DE-A 37 01 547) or polybutylacrylates (EP-A 422 533) as auxiliaries impart to the finished coatingmaterials the abovementioned, disadvantageous properties, in a more orless pronounced form.

The object of the present invention was to find new binder systems basedon basic, thermally crosslinkable epoxy or polyurethane resins asbinders, which exhibit a good behavior in respect of both the surfacequality of the cathodically deposited coatings and the adhesion oftopcoats applied subsequently.

To this end the binder systems defined at the outset were found.

Also found were new pigment paste formulations for electrodepositioncoating, a method of electrodeposition coating using the new bindersystems, and the use of polymers (I) as an additive to electrodepositionbaths for improving the surface quality of the articles coated by meansof electrodeposition, and the coated articles themselves.

The binder systems according to the invention contain from 0.001 to 1%by weight, preferably from 0.01 to 0.75% by weight, of a polymer (I)whose predominant components are ethyl acrylate and/or n-propylacrylate, based on the overall solids content of the binder system. Theterm "predominant components" should be understood as referring to thosequantities of ethyl and/or n-propyl acrylate whose content in a polymer(I) is more than 50 mol %. Particularly suitable polymers (I) arehomopolymers of ethyl acrylate and of n-propyl acrylate and copolymersof these two monomer building blocks.

Other suitable polymers (I) are those containing minor quantities ofother olefinically unsaturated monomers. Their proportion, based on thepolymer (I), is generally less than 50 mol %, preferably from 0.1 to mol%. Examples of comonomers which can be used are the following:

monomers containing amino groups,

e.g. N-dialkylaminoalkyl acrylates and methacrylates such as

N-dimethylaminoethyl acrylate

N-dimethylaminoethyl methacrylate

N-diethylaminoethyl acrylate

N-diethylaminoethyl methacrylate

N-dimethylamino-n-propyl acrylate

N-dimethylamino-n-propyl methacrylate

N-diethylamino-n-propyl acrylate and

N-diethylamino-n-propyl methacrylate

and N-dialkylaminoalkylacrylamides and -methacrylamides such as

N-dimethylaminoethylacrylamide

N-dimethylaminoethylmethacrylamide

N-diethylaminoethylacrylamide and

N-diethylaminoethylmethacrylamide

in which context N-alkanol-substituted derivatives of aminoalkylacrylates and methacrylates and aminoalkylacrylamides and-methacrylamides, and vinyl-substituted heterocycles containing tertiaryamino functions, such as N-vinylimidazole, are also suitable;

monomers containing hydroxyl groups, e.g. hydroxyalkyl acrylates andmethacrylates such as

2-hydroxyethyl acrylate

2-hydroxyethyl methacrylate

3-hydroxypropyl acrylate

3-hydroxypropyl methacrylate

4-hydroxybutyl acrylate and

4-hydroxybutyl methacrylate

alkyl acrylates such as

i-propyl acrylate

n-butyl acrylate

i-butyl acrylate

n-pentyl acrylate and

n-hexyl acrylate

alkyl methacrylates such as

ethyl methacrylate

n-propyl methacrylate

i-propyl methacrylate

n-butyl methacrylate

i-butyl methacrylate

n-pentyl methacrylate

n-hexyl methacrylate and

2-ethylhexyl methacrylate

The polymers (I) generally have an average molecular weight of between5000 and 50,000, preferably between 500 and 25,000.

The preparation of I is known per se and can be carried out in aconventional manner by free-radical polymerization, for example bysolution polymerization at a temperature of from 50° to 200° C., inparticular from 70° to 150° C. Examples of suitable solvents aren-butanol, isobutanol, sec-butanol, tert-butanol, dioxane,tetrahydrofuran, acetone, methoxypropanone, methoxy-, ethoxy- andn-butoxyethanol and diethylene glycol dimethyl ether, ethylene,propylene and diethylene glycol, with sec-butanol being particularlysuitable. Other suitable solvents are cyclohexane, benzene, toluene andxylene.

Free-radical initiators which can be used are the conventional initiatorsubstances such as 1,1'-azobisisobutyronitrile,1,1'-azobiscyclohexanecarbonitrile, 2,2'-azobis(2-cyanopropane), benzoylperoxide, di-tert-butyl peroxide, tert-butyl hydroperoxide, cumenehydroperoxide and tert-butyl perbenzoate and, in particular, tert-butylperoctanoate, the quantity of initiator advantageously being from 0.1 to5% by weight, preferably from 0.5 to 3% by weight, based on the quantityof monomer employed.

The initiator is generally added as a solution, either in the monomersor, preferably, in the solvent.

In addition, the polymers (I) can be prepared by anionic polymerizationusing appropriate organolithium compounds as initiators, for examplen-butyllithium or methyllithium, in solvents such as diethyl ether ortetrahydrofuran.

The polymers (I) are generally used as dispersions. They can also beused as a solution in the solvents which come from the polymerization.

The dispersions of I are conventionally prepared such that they have asolids content of from 10 to 70% by weight, preferably from 30 to 55% byweight.

The binders (B) in the binder systems according to the invention arebasic, thermally crosslinkable polymers of the epoxy and polyurethaneresin type. Suitable binders--also called base resin--are thosematerials which carry groups which can be protonated with acids, forexample amino or thiol groups. After protonation these groups ensure thedispersibility of the resins in water. Particularly suitable resins arethose containing primary and/or secondary amino groups. Examples ofresins containing amino groups are amino-epoxy and amino-polyurethaneresins.

All base resins preferably have an average molecular weight M_(w),measurable in the conventional way by means of gel permeationchromatography, of from 200 to 20,000, particularly preferably from 1000to 15,000. The total number of acid-protonatable groups is on averagepreferably from 1 to 10, particularly preferably from 2 to 7, perpolymer resin chain.

In addition to the polyurethane resins, particularly preferred baseresins are those whose basic structures are based on epoxy resins,especially those containing terminal epoxide groups. Particularlysuitable epoxy resins are materials containing on average from 1 to 3,preferably from 1.8 to 2.2, epoxide groups per molecule and having anaverage molecular weight of from 100 to 10,000. Preferred epoxy resinshave an average molecular weight M_(w) of from 150 to 5000, inparticular from 200 to 3500.

Suitable epoxy resins are the reaction products, obtainable in aconventional manner, of polyhydric phenols with epihalohydrins in thepresence of a base.

Examples of polyhydric phenols are:

resorcinol

hydroquinone

4,4'-dihydroxybenzophenone

4,4'-dihydroxybiphenyl

bis(4-hydroxyphenyl)methane

1,1-di(4-hydroxyphenyl)ethane

2,2-di(4-hydroxyphenyl)propane (bisphenol A)

1,1-di(4-hydroxyphenyl)isobutane

1,5-dihydroxynaphthalene and

novolaks

Bisphenol A is particularly preferred.

The preferred epihalohydrin is epichlorohydrin. Furthermore,epihalohydrins can also be replaced in whole or in part by polyetherswhich carry terminal epoxide groups. Examples of these are polyetherscomposed of ethylene oxide, propylene oxide and tetrahydrofuran.

Resins which can be used in addition to the epoxy resins comprisingpolyhydric phenols and epihalohydrins are also reaction products ofhalohydrins and polyhydric aliphatic alcohols such as ethylene glycol,diethylene glycol, triethylene glycol, 1,2-prepylene glycol,1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,2,6-hexanetriol, glycerol and 2,2-di(4-hydroxycyclohexane)propane.Very particularly preferred epoxy resins are those which can be obtainedby reacting polyhydric phenols, especially bisphenol A, with diglycidylethers of bisphenol A. The reaction is usually carried out in thepresence of, for example, propylene glycol phenyl ether, xylene orisobutanol as solvent at a temperature of from 50° to 200° C., inparticular from 100° to 180° C. The reaction is generally catalyzed bytriphenylphosphine or dimethylbenzylamine.

The basic epoxy resins which are suitable as binders are, in particular,amino-epoxy resins which can be obtained by reacting epoxidegroup-containing resins--preferably with terminal epoxide groups--withan aliphatic amine which carries one or more other functional groups,such as amino groups and/or hydroxyl groups. It is recommended in thiscontext to employ the amine in excess. Aliphatic amines includealkylenediamines, alkanolamines, polyoxyalkylenepolyamines andpolyfunctional polyolefin amines.

Suitable such compounds are:

alkylenediamines having two primary amino groups and from 2 to 20 carbonatoms in the alkylene radical, especially ethylenediamine, 1,2- and

1,3-diaminopropane, 1,4-diaminobutane, neopentanediamine andhexamethylenediamine

alkylalkanolamines having in each case from 1 to 20 carbon atoms in thealkyl and alkanol radical, in which the chain length of the radicals maybe identical to or different from one another, e.g. ethylethanolamine,methylisopropanolamine and especially methylethanolamine

polyoxyalkylenepolyamines, e.g. polyoxyethylene-polyamine,polyoxypropylenepolyamine and polyoxybutylenepolyamine

polyolefinamines, such as amine-terminal butadiene-acrylonitrilecopolymers having an average molecular weight of from 1000 to 10,000

Also suitable are alkylamines, including in particular dialkylaminessuch as dimethylamine and diethylamine. If desired, the amino-epoxyresins can be chain-extended using dicarboxylic acids, for examplesebacic acid or a dimeric fatty acid. The conjoint use of monocarboxylicacids, such as a C₁₂ to C₂₄ fatty acid, is also possible.

Furthermore, the amino-epoxy resins can be used in a mixture with up to20% by weight of a polyester. The latter are polycondensation productsof dicarbcxylic acids and polyhydric alcohols. They have an averagemolecular weight of from 200 to 20,000, preferably from 200 to 5000.

Suitable carboxylic acid components are compounds such as maleic acid,succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, cyclohexanedicarboxylic acid, phthalic acid,isophthalic acid and terephthalic acid, and functional derivatives ofthese acids.

Alcohol components which may be mentioned are aliphatic diols such asethylene glycol, diethylene glycol, triethylene glycol, propanediol,butanediol, hexanediol, neopentylglycol and neopentylglycolhydroxypivalate, and also more highly functional alcohols such astrimethylolpropane, glycerol, erythritol, pentaerythritol,trimethylolbenzene and tris(hydroxyethyl) isocyanurate.

Amine components are preferably introduced into the epoxy resins byreaction with so-called amidoamines, i.e. condensation products ofpolyamines, preferably aliphatic polyamines such as1,6-hexamethylenediamine, diethylenetriamine and triethylenetetramine,in which context 1,6-hexamethylenediamine is particularly preferred, anddicarboxylic acids, preferably dimer fatty acids. Carboxy-terminalmonocarboxylic acids, preferably C₁₂ -C₂₀ -carboxylic acids, can also beincorporated into the amidoamine.

The reaction of the epoxy resin with the amine component is preferablycarried out at from 50° to 90° C. in a polar solvent, such as isobutanoland sec-butanol, which is generally present in quantities of from 5 to50% by weight of the batch. The reaction is generally at an end aftertwo hours.

Other resins which have proven suitable as base resins are basicpolyurethane resins having an average molecular weight M_(w) of from 200to 10,000 made from aliphatic and/or aromatic diisocyanates andaliphatic diols or polyols. Diisocyanates which may be mentioned inparticular are tetramethylene diisocyanate, 1,6-hexamethylenediisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate,tolylene diisocyanate, naphthylene diisocyanate and 4,4'-diphenyl etherdiisocyanate.

Particularly suitable diols are ethylene glycol, diethylene glycol,triethylene glycol, propanediol, butanediol, hexanediol, neopentylglycoland hydroxypivalic acid neopentylglycol [sic]. However, it is alsopossible to use more highly functional alcohols such astrimethylolpropane, glycerol, pentaerythritol and trimethylolbenzene.

Among the basic polyurethane resins, amino-polyurethane resins arepreferred as base resin. By reacting the isocyanates withamino-terminal, polyfunctional amines it is possible to introduceprimary and secondary amino groups into the resin. Suitable amines arecompounds containing primary and secondary amine functions, such asdiethylenetriamine, triethylenetetramine, tetraethylenepentamine andpolyetherdiamines having terminal amino groups. Also suitable,furthermore, are amines having primary and tertiary amine functions, forexample dimethylaminopropylamine and diethylaminopropylamine.

Amino groups can be introduced into the polyurethane resin by one of theconventional methods, as are described in, for example, U.S. Pat. No.4,016,120.

The basic epoxy and polyurethane resins can be converted into awater-soluble or water-dispersible form by protonation with acid. Acidssuitable for the partial or complete neutralization are organic acidssuch as formic acid, acetic acid and lactic acid, but also mineral acidssuch as phosphoric acid.

For the majority of intended applications, for example those ofelectrodeposition coating, the binder systems according to the inventioncontain as additional component a crosslinking agent (V), preferably ina quantity of from 10 to 50% by weight, particularly preferably from 25to 40% by weight.

The crosslinking agents are in general polyfunctional, monomeric orpolymeric compounds which, under the effect of heat, have a crosslinkingaction via condensation and addition reactions.

Particularly suitable crosslinking agents are blocked isocyanates ormixtures of various isocyanates, and phenolic Mannich bases.

Examples of crosslinking agents of the isocyanate type are1,6-hexamethylene diisocyanate, naphthalene diisocyanates andtriphenylmethane triisocyanates, and the trimers of 1,6-hexamethylenediisocyanate, isophorone diisocyanate and tolylene diisocyanate.Tolylene diisocyanate and isophorone diisocyanate are preferred. Thetrimer of 1,6-hexamethylene diisocyanate is particularly preferred.

Examples of suitable blocking agents are monohydric alcohols, preferablyshort-chain aliphatic alcohols such as methanol, ethanol, n-propanol,isopropanol, n-butanol, isobutanol and sec-butanol. Also suitable aremonoethers of aliphatic diols, such as ethylene glycol monopropyl etherand ethylene glycol monobutyl ether. Other suitable compounds aresecondary amines, preferably short-chain aliphatic amines such asdimethylamine, diethylamine, dipropylamine and dibutylamine, andalkanolamines, especially tertiary alkanolamines such astri(n-propanol)amine and tri(isopropanol)amine, or mixtures thereof. Thepolyfunctional isocyanates are reacted with the blocking compounds in amanner known per se in quantities such that, on average, all--as far aspossible--isocyanate groups per molecule are blocked.

The crosslinking agents of the phenolic Mannich base type known fromDE-A 34 22 457 are aminomethylated phenols which are obtained byreacting polycyclic phenols with formaldehyde and secondary amines.

Examples of other crosslinking agents which can be used are amino resinssuch as urea, formaldehyde, melamine and benzoguanamine resins, andcrosslinking agents which cure by means of transesterification and esteraminolysis, for example β-hydroxyalkyl esters according to EP-A 40 867and carboalkoxymethyl esters according to DE-A 32 33 139.

It is advantageous to disperse the crosslinking component (V) togetherwith the base resin (B). This can be carried out by mixing (B) and (V),neutralizing the mixture with acid and subsequently dispersing it inwater. The mixture of (B) and (V), which generally contains from 50 to90% by weight of B and from 10 to 50% by weight of V, can also be addedto an acid/water mixture.

The conjoint dispersion of (B) and (V) is conventionally adjusted tosolids contents of from 15 to 45% by weight.

In general, for the preparation of electrodeposition baths, the bindersystems according to the invention which contain the polymer (I), thebinder (B) and the crosslinking agent (V) have pigment pastes added tothem.

The pigment pastes contain, in addition to the conventional pigmentssuch as titanium dioxide, carbon black and barium sulfate, in generalbinders, which advantageously are in accordance with the basic resin,and also solvents, fillers such as chalk and sodium silicates, and paintauxiliaries, for example corrosion inhibitors, antifoams, dispersionassistants and curing catalysts.

The pigment pastes are prepared by the conventional methods. The solidscontent of the pigment pastes is in general from 40 to 70% by weight.

Pigment pastes containing a polymer (I) as binder component areparticularly preferred.

In these pigment paste formulations according to the invention,comprising pigments, binders, solvents, fillers and other paintauxiliaries, from 0.0005 to 0.5% by weight, in particular from 0.1 to0.4% by weight, of a polymer (I) is present.

These pigment paste formulations are prepared, in accordance with thepreparation of the pigment pastes, with the further addition of apolymer (I). The pigment paste formulations according to the inventionhave a solids content of from 40 to 70% by weight.

For the preparation of the aqueous electrodeposition coating baths,either an already finished pigment paste formulation according to theinvention or, in each case as individual component, a polymer (I) and apigment paste can be added to a dispersion of (B) and (V). Forelectrodeposition coating, the deposition baths are generally adjustedto a solids content of from 10 to 30% by weight.

The binder systems according to the invention and the pigment pasteformulations according to the invention are preferably used forelectrodeposition coating, in which context the polymers (I) which theycontain, whose predominant components are ethyl acrylate and/or n-propylacrylate, are used as an additive to electrodeposition baths forimproving the surface quality of the articles coated by means ofelectrodeposition.

The cathodic electrodeposition coating is carried out by a knownprocedure. The deposition of the coating material takes placeconventionally at temperatures of between 20° and 35° C. over a periodof from 1 to 5 minutes and at a pH of between 5 and 8, preferably ataround the neutral point. The deposition voltages are generally between50 and 500 volts. The electrically conductive articles to be coated, forexample degreased deep-drawn metal panels, phosphatized and galvanizedsteel panels, are connected as cathode. The deposited coating film isadvantageously cured at temperatures above 120° C., preferably between140° and 180° C., over a period of from approximately 10 to 45 minutes.Under these conditions of electrodeposition coating it is possible toproduce coating films having a film thickness of from 10 to 35 μm.

Coating films produced by deposition from electrodeposition bathscontaining the binder systems according to the invention exhibit a goodsurface quality of the coated articles, i.e. few coating flow problemsand little formation of craters, coupled with good adhesion propertiesof the coats applied subsequently, such as topcoats.

EXAMPLES

1. Preparation of a polymer from ethyl and n-propyl acrylate (I)

A mixture of 220 g (2.2 mol) of ethyl acrylate and 250.8 g (2.2 mol) ofn-propyl acrylate was added over the course of two hours under anitrogen atmosphere to 356 g of boiling sec-butanol. In paralleltherewith, a solution of 5.27 g (25 mmol) of tert-butyl peroctanoate in115 g of sec-butanol was added over a period of 3.5 hours. Subsequently,polymerization was carried out for a further three hours at atemperature of 75° C. The solids content of the reaction solution was50%.

K value: 18.8 (3% strength solution of I in acetone)

Surface tension: 30 mN/m

M_(w) : 13,750 g/mol

2. Preparation of an amino-epoxy resin (binder B)

4850 g of a diglycidyl ether made from bisphenol A and epichlorohydrin(epoxide equivalent weight 485) were dissolved at 65° C. in 1039 g oftoluene and 1039 g of isobutanol. After this solution had been cooled to60° C., a solution of 300.4 g (4 mol) of methylethanolamine in 128 g ofisobutanol was added. After addition of 1850 g of a solution of anamidoamine, the mixture was heated at 80° C. for 2 hours. The resin hada solids content of 70% and an amine number of 197 mg of KOH per g ofresin. The number of acid-protonatable groups and of hydroxyl groups wason average 1.8.

The amidoamine was prepared by reacting 5800 g (50 mol) of1,6-hexamethylenediamine, 7250 g (12.5 mol) of dimeric fatty acid and1400 g (5 mol) of linseed oil fatty acid at 195° C., with distillativeremoval of the water of reaction. After the mixture had been cooled to100° C. and diluted with 5961 g of toluene, the amine number was 197 mgof KOH per g of resin.

3. Preparation of a crosslinking agent (V)

5042 g (10 mol) of trimerized 1,6-hexamethylenediamine in 3823 g ofmethyl isobutyl ketone were admixed at 70° C. with 3881 g (30 mol) ofdi-n-butylamine, and the mixture was maintained at this temperatureuntil virtually no further isocyanate could be detected. The solidscontent was 70%.

Preparation of a binder/crosslinking agent mixture (B/V)

700 g of amino-epoxy resin (B) and 300 [lacuna] crosslinking agent (V)were admixed with 19 g of acetic acid and dispersed in a quantity ofwater such that the resulting dispersion had a solids content of 31%.The organic solvents were removed by azeotropic distillation. Thedispersion was adjusted to a solids content of 35% with water.

5. Preparation of a pigment paste (PP)

525.8 g of amino-epoxy resin (B) together with 16.5 g of acetic acidwere dispersed in 168.7 g of butylglycol and 600 g of water. After theaddition of 800 g of titanium dioxide, 11 g of carbon black and 50 g ofbasic lead silicate the mixture was ground in a ball mill to a particlesize of less than 3 μm. The millbase was subsequently adjusted to asolids content of about 50% with water.

6. Preparation of a pigment paste formulation according to theinvention, PP/I, from PP and polymer I

This pigment paste formulation PP/I was prepared in accordance withExample 5 with the further addition of 10.1 g of polymer I, bydispersion.

Solids content: 49%

7. Preparation of electrodeposition coating dispersions

D1: Preparation of an aqueous dispersion of B/V, I/1 and PP

1095 g of the mixture B/V according to Example 4 were mixed with 10.1 gof the polymer made from ethyl and n-propyl acrylate (I) from Example 1.After addition of 20.7 g of acetic acid, the mixture was dispersed in1200 g of water. The organic solvents were removed by azeotropicdistillation. The resulting dispersion was admixed with 568 g of thepigment paste PP from Example 5, and was stirred and made up to adispersion quantity of 5000 g with water.

Solids content: 21%

D2: Preparation of an aqueous dispersion of PP/I and B/V

573 g of pigment paste PP/I according to Example 6 were mixed with 1964g of the mixture B/V according to Example 4, and the mixture was made upto a dispersion quantity of 5000 g with water.

Solids content: 21%

D3: Preparation of an aqueous dispersion of B/V and PP (without polymerI)

1095 g of the mixture B/V according to Example 4 were admixed with 20.7g of acetic acid and dispersed in 1200 g of water. The resultingdispersion had a solids content of 34%. The dispersion obtained wasadmixed with 568 g of pigment paste PP according to Example 5, and wasstirred and made up to a dispersion quantity of 5000 g with water.

Solids content: 21%

The aqueous electrodeposition coating dispersions D1-D3 were aged atroom temperature for 5 days with stirring.

Applications testing

Coating films were deposited conventionally on phosphatized steel panelsconnected as cathode at 27° C. over a period of 2 minutes, and werebaked at a temperature of 160° C. for 20 minutes. At deposition voltagesof from 350 to 430 V, film thicknesses of from 22 to 24 μm wereachieved.

The quality of the deposited coating films was determined on the basisof six investigations. Assessment was in each case visual.

The following investigations were carried out:

(a) Bridging at panel seams and flanges

This test examines the extent to which bridges of coating materialdevelop at panel seams and flanges of interconnected workpieces. Forthis purpose test panels which following coating deposition retainresidues of coating material in the panel seams and flanges aresubjected to a washing procedure with desalinated water. In thisprocedure, however, the residues of coating material are not alwayscompletely removed, so that during the subsequent baking procedurebridges of coating material are formed from the residues which remain.

    ______________________________________                                        Assessment:   0 ≅                                                                           no bridging                                                         5 ≅                                                                           extensive bridges of                                                          coating material                                      ______________________________________                                    

(b) Coating material runs at panel seams and flanges

This test is carried out in accordance with (a) on workpieces standingvertically. During the baking procedure the residues of coating materialnot removed completely by the washing procedure run out of the panelseams and flanges, since the viscosity of the coating material decreasesinitially because of the rising temperature. The subsequent rise inviscosity at the crosslinking temperature of the coating material leadsto the formation of runs--also called paint noses.

    ______________________________________                                        Assessment:    0 ≅                                                                            no marks                                                           5 ≅                                                                            extensive paint                                                               noses                                               ______________________________________                                    

(c) Oil-splash sensitivity and contamination resistance

Substances such as fats and silicone oils can pass from the workpiecesto be coated into the electrodeposition bath, and reduce the surfacetension of the coating material. During the deposition coating procedurethey are deposited together with the coating material. During baking,substances of this kind present on and in the coating film bring about apaint flux. These flux phenomena have their beginnings at the pointswhere the interfering substances are deposited, and have the appearanceof craters in the baked coating film.

    ______________________________________                                        Assessment:   0 ≅                                                                           no effect                                                           5 ≅                                                                           extensive cratering                                   ______________________________________                                    

(d) Sensitivity to water-spotting

This test assesses plastic marks caused during the baking procedure bythe boiling of water drops.

After deposition of the coating material the workpieces are washed withdesalinated water in a spraying zone and pass, attached to a suspensiondevice, through an oven zone for the baking procedure. Drops of waterwhich fall from this device onto the washed coating surface, and tracesof water remaining on the coating from the prior washing procedure, canbring about--through boiling--plastic marks on the coating film duringbaking, for example in the form of small bubbles.

    ______________________________________                                        Assessment:    0 ≅                                                                           no marks                                                            5 ≅                                                                           severe boil marks                                    ______________________________________                                    

(e) Salt spray test SST (in accordance with DIN 50017)

This test serves to assess the adhesion properties of topcoats. For thispurpose the untreated and slit panels are exposed for 240 hours at 35°C. to a 5% strength salt solution, which is applied as a fine mist in aspray chamber. Subsequently the extent of under-rusting at the slit ismeasured in mm (in accordance with DIN 50017), averaged over 5 samplepanels.

(f) Topcoat adhesion test

Crosshatch cuts passing through the coating to the surface of the panelare carried out on the coated test panels. The adhesion of the topcoatis tested using adhesive strips. The quantitative delamination of thecoating is assessed visually using the following percentage gradings.

    ______________________________________                                        Assessment:   0 ≅                                                                          no delamination                                                      1 ≦ 5%                                                                          "                                                                    2 ≦ 15%                                                                         "                                                                    3 ≦ 35%                                                                         "                                                                    4 ≦ 65%                                                                         "                                                                    5 > 65%  "                                                      ______________________________________                                    

The same investigations are carried out on coated test panels which havebeen exposed beforehand for 240 hours to a salt-containing atmosphere inaccordance with DIN 50 017 (salt spray test, (e)).

The results are compiled in the table.

                  TABLE                                                           ______________________________________                                                       Dispersions                                                                               D3                                                                D1  D2      (without polymer I)                                ______________________________________                                        (a) Bridging at panel                                                                          4     3       5                                              seams and flanges                                                             (b) Runs at panel seams                                                                        3     3       5                                              and flanges                                                                   (c) Oil-splash sensitivity                                                                     3     2       4                                              and contamination                                                             resistance                                                                    (d) Sensitivity to                                                                             3     3       4-5                                            water-spotting                                                                (e) Salt spray test                                                                            1.4   1.5     1.4                                            (DIN 50017)                                                                   Extent of under-rusting                                                       at the slit (mm)                                                              (f) Topcoat adhesion test                                                                      1/2   1/2     1/2                                            before/after 240 h salt                                                       spray test                                                                    ______________________________________                                    

We claim:
 1. A cathodic electrodeposition composition comprising abinder system, said binder system comprising:a) at least one basic,thermally crosslinkable resin selected from the group consisting ofepoxy resins, polyurethane resins, and mixtures thereof, and b) from0.001 to 1% by weight, based on the overall solids content of the bindersystem, of an acrylic polymer that is a copolymer consisting essentiallyof monomers selected from the group consisting of ethyl acrylate,n-propyl acrylate, and mixtures thereof.
 2. A composition according toclaim 1, wherein the acrylic polymer of (b) is present at a level ofabout from 0.01 to 0.75% by weight of the binder system.
 3. Acomposition according to claim 1, wherein the crosslinkable resin has anaverage molecular weight M_(w) of from about 200 to 20,000.
 4. Acomposition according to claim 1, wherein the crosslinkable resincomprises an epoxy resin obtained by reacting2,2-di(4-hydroxyphenyl)propane, epichlorohydrin, and an aliphatic amine.5. A method of cathodic electrodeposition coating, comprising the stepsof:a) preparing an electrodeposition coating composition according toclaim 1; b) immersing an article to be coated as cathode into thecathodic electrodeposition coating composition; c) passing a depositionvoltage between about 50 to 500 volts over a period of from about 1 to 5minutes to deposit a coating on the article; removing the article; ande) curing the deposited coating.
 6. A coated article comprising acoating that includes a composition according to claim
 1. 7. A pigmentpaste composition suitable for cathodic electrodeposition coating,comprising compounds selected from the group consisting of pigments,binders, solvents, fillers, and other paint auxiliaries; and whereinsaid pigment paste composition includes from 0.0005 to 0.5% by weight,based on the overall solids content of the pigment paste composition, ofan acrylic polymer that is a copolymer consisting essentially ofmonomers selected from the group consisting of ethyl acrylate, n-propylacrylate, and mixtures thereof.
 8. A method of cathodicelectrodeposition coating, comprising the steps of:a) preparing anelectrodeposition coating composition; b) adding to theelectrodeposition coating composition a pigment paste compositionaccording to claim 7; c) immersing an article to be coated as cathodeinto the cathodic electrodeposition coating composition; d) passing adeposition voltage between about 50 to 500 volts over a period of fromabout 1 to 5 minutes to deposit a coating on the article; e) removingthe article; and f) curing the deposited coating.
 9. A method forimproving the surface quality of a cured film obtained from a cathodicelectrodeposition coating, comprising preparing a cathodicelectrodeposition coating composition comprising a binder system, saidbinder system comprising:a) at least one basic, thermally crosslinkableresin selected from the group consisting of epoxy resins, polyurethaneresins, and mixtures thereof, and b) from 0.001 to 1% by weight, basedon the overall solids content of the binder system, of an acrylicpolymer that is a copolymer consisting essentially of monomers selectedfrom the group consisting of ethyl acrylate, n-propyl acrylate, andmixtures thereof.